1. Field of the Invention
The present invention relates to a method by which two optical fibers, each including a bare fiber surrounded by a thin coating layer made of a carbon-based material such as titanium carbide or silicon carbide which in turn is covered with a jacket, are joined together by fusion.
2. Description of the Prior Art
FIG. 3(a) shows a conventional optical fiber 20. FIG. 3(b) shows the optical fiber 20 with the end of its jacket 22 removed to expose the bare fiber 21.
As shown in FIG. 3(b), a conventional method of joining two optical fibers having the construction described above comprises removing the end of each jacket 22 to expose the bare fiber 21, wiping dust particles and any other foreign matter from the bare fiber, making the ends of the bare fibers face each other, achieving optical alignment between the cores 21' of the fibers 20 by image processing, and fusing them together by the heat of an electric air discharge.
Optical alignment could be achieved conveniently by matching the outside diameters of optical fibers 20. However, if the core 21' of one of the optical fibers 20 is off-centered, the resulting joint will be as shown in FIG. 4(a).
An optical signal transmitted through the core 21' will be scattered thereby incurring excessive loss.
If, on the other hand, two optical fibers 20 with eccentric cores are joined together with the cores 21' being aligned by means of a direct core viewing fusing splicer, the resulting joint will be as shown in FIG. 4(b). There is a slight mismatch between the circumferences of the two optical fibers but their cores are in complete alignment so as to minimize the loss at the joint.
Recent advances in optical fiber technology have led to the development of high-quality optical fibers that have stable transmission characteristics and improved long-term reliability of their mechanical strength.
FIG. 5(a) and 5(b) show an example of such an improved optical fiber 10. FIG. 5(b) shows the optical fiber with the jacket removed at the end thereof. As shown, a bare optical fiber 11 is surrounded by a thin coating layer 12 having a thickness of 250-1000 .ANG. which is made of a carbon based material such as titanium carbide or silicon carbide, which in turn is covered with a jacket 13 made of a material such as plastic.
Unlike the optical fiber 20 shown in FIG. 3, the optical fiber 10 has the carbon-based thin coating layer 12 formed on the circumference of the bare fiber 11. This is remarkably effective in preventing both changes in the chemical or physical properties of the optical fiber and the deterioration of the fiber's mechanical strength.
However, if one attempts to join two optical fibers 10 having the carbon-based thin coating layer 12 by means of a direct core viewing fusion splicer without incurring excessive loss at the joint, the following problems occur. The first problem is associated with the operating principle of the direct core viewing fusion splicer. To operate this machine, the ends of the bare optical fibers 11 to be joined are brought face-to-face with one another and light is projected from the lateral side of each fiber. The transmitted light produces a boundary between the bare optical fiber 11 and the core 11' on account of the difference between their refractive indices. The image of the boundary is observed on a CRT as shown in FIG. 6(a) so as to achieve axial alignment between the cores of the two optical fibers. However, the optical fiber 10 having the carbon-based thin coating layer 12 does not permit transmission of the incident light and the limits of the core 11' cannot be discerned. The resulting image on the CRT will be as shown in FIG. 6(b). Thus, the proper joint can not be produced..
Secondly, it is possible to join two optical fibers 10 by means of a direct core viewing fusion splicer if the two fibers are forced into alignment with respect to their circumference. In this case, though, the following two problems will arise. First, when the ends of the two optical fibers are placed face-to-face with one another are fused together by electric discharge, a part of the carbon-based thin coating layer 12 will char from the heat of discharge and get into the fused ends of the optical fiber, potentially causing an increased transmission loss and deterioration of strength at the joint. Secondly, the core 11' may be off-centered with respect to the circle defined by the circumference of the bare fiber 11. In the sense of such eccentricity, a low-loss joint can not be ensured even if the circumference of one fiber is aligned with that of the other fiber.